1. Field of the Invention
This invention relates generally to a light projection apparatus and, more particularly, but not exclusively, to an apparatus for use in the projection of television or video pictures and similarly derived images of computer generated or other visual information onto large screens.
2. Description of the Prior Art
Though attempts have been made to produce a commercially acceptable laser large screen color projection apparatus, the primary problem with the display technologies that use lasers as a light source has been the inadequate display brightness as compared to manufacturing and/or operating costs. This is due to the fact that prior devices did not fully utilize all of the available light output from two lasers to produce a properly balanced color display.
Though various individual components of such an improved system have been available for some time, a brighter color display achieved through proper balanced and selected combination of the red, green and blue light beam components of two or more lasers has not been achieved in the industry. Some of these components necessary for a light projection apparatus have been disclosed by Charles E. Baker in Texas Instruments Inc.'s paper published in the I.E.E.E. Spectrum in December 1968. Also, certain components were disclosed in the 1970 article entitled, Funkschau, 1970, Heft 4, Farbfernseh-Gro.beta.projektion Mit Laser.
FIG. 1 discloses a typical prior art arrangement using two lasers to produce a light beam that is scanned onto a viewing surface. In FIG. 1, a first Argon ion laser transmits a light beam that is reflected 90.degree. by a mirror R.sub.1 to a dye laser. The Argon ion lasers in FIG. 1 produce blue, green and blue/green light beam components that contain all of the wavelengths required for full color video displays except for the red light beam component. Therefore, a dye laser is required to be used in combination with the Argon laser to produce the red light beam component. This dye laser produces a red light beam component reflected by mirror R.sub.2 that is transmitted through modulator M.sub.1 and lens L.sub.1 and then transmitted to the scanning means S.sub.1 and S.sub.2 which projects the light from S.sub.2 onto a viewing surface. The second Argon ion laser produces a light beam that is reflected 90.degree. by mirror R.sub.3 to a dichroic beam splitter D.sub.1 which permits straight-line transmission of the blue-green and green light beam components but reflects the blue light beam component 90.degree. to a second modulator M.sub.2 and lens L.sub.2 which is then in turn reflected by mirror R.sub.5 to the scanning means for projection onto the viewing surface. The remaining straight-line light beam components are transmitted to a second dichroic beam splitter D.sub.2 which reflects the green light beam component 90.degree. to its modulator M.sub.3 and lens L.sub.3 then onto the scanning means to the viewing surface. The remaining blue-green light beam component not reflected to modulator M.sub.3 is transmitted to beam stop B.sub.1.
Another prior art device using a laser and a dye laser is found in U.S. Pat. No. 4,613,201 which discloses the use of a single Argon laser which produces a light beam. The blue light beam component is separated from the other wavelengths using a dispersing prism P.sub.1, as seen in FIG. 2. The blue light beam component is reflected to its modulator M.sub.2 and then transmitted to the scanning means for projection onto a viewing surface. All of the remaining wavelengths are transmitted to a polarizing prism P.sub.2 which reflects a portion of the beam for the green light beam component 90.degree. while transmitting the remaining portion to the dye laser. The green light beam component and red light beam component are passed through their respective modulators M.sub.3, M.sub.1 and then onto the scanning means for projection onto the viewing surface, as best seen in FIG. 2.
The production of the red light beam component is currently achieved using a diode laser, krypton ion laser or a dye laser. Diode lasers do not provide enough power at the required wavelength. The red light component produced by the krypton ion laser requires four-to-five times the power as the comparable power of an Argon ion laser. Also, the blue and green light beam components of the krypton laser are quite weak compared to its red component. The krypton red light component is at a wavelength that the human eye is not as sensitive to and therefore makes it difficult to balance with the other colors to give a complete color scale with reasonable power.
The Argon ion laser in combination with a dye laser is therefore preferred in providing the blue-green-red light beam components. The dye laser preferably converts light energy of a shorter wavelength to a longer, tuneable wavelength.
These above light projection devices, while providing a viewable display on a large screen, have not provided a desirable balanced color display. For example, when using the single Argon ion laser of U.S. Pat. No. 4,613,201, as illustrated in FIG. 2, the 454-476 nm blue, generally defined as the blue light beam component, would be taken out of the remaining light beam transmitted to the polarizing prism P.sub.2.
The polarizing prism P.sub.2 is utilized to divide the remaining beam containing 488 nm and 514 nm into two beams; one for the green beam and the other for the dye pump beam. The ratio of the beam intensities is adjustable by varying a stress plate positioned just before P.sub.2. This optic will vary the polarization of the beam and P.sub.2 will then divide the beams based on the polarization ratios. This method will vary the intensities of the transmitted and reflected beams but is not wavelength selective. The ratio of 488 nm and 514 nm will remain the same and both wavelengths will be present in both beams. Allowing 488 nm into the green beam will diminish the color distinction between blue and green and not provide a proper color balanced image or display. This one laser layout will also not allow for full use of all lines due to the fact that if the green output is adjusted to match the blue output there will be an excess of red output or if the red output is adjusted to the blue output there will be an excess of green output.
In the two laser system, as shown in FIG. an Argon ion laser is used for the blue and green light beam components and another Argon laser is used only for supplying the energy for the dye laser and subsequently the red light beam component. In this system, there is more than an adequate amount of 514 nm green (green light beam component) for a balanced color display as compared to the amount of 454-476 nm blue (blue light beam component) and 610 nm red (red light component).
More particularly, when using the two Argon ion lasers and a dye laser system of FIG. 1, all the 514 nm green and 488 nm to 501 nm blue-green, generally defined as the blue-green light beam component, of one of the lasers participates in producing the red light beam component but the blue light beam component (454-476 nm) is wasted since it does not significantly contribute to the production of the red component in a dye laser. In the other Argon ion laser, the 454-476 nm blue is used for the blue light component and 514 nm green is used for the green light beam component; however, the blue-green light beam components are either not produced using special optics in the laser or are separated and dissipated. Since a balanced color display requires a Green:Blue:Red ratio of approximately 1:1:1.1 and the typical two laser system produces a ratio of 7:3:4, the two laser system produces greater than twice as much green as blue, and the excess green is lost.
Therefore, there has been a long felt need in the industry to provide the highest light output power competitive with other large screen image projection technologies by fully utilizing all of the available power output from two lasers.
Also, the prior art light projection systems have undesirably positioned the two lasers on each side, or in the case of a single laser system to one side, of the central location usually occupied by the lenses/optics, modulators, mirrors, beam splitters and other components required in a light projection system. This inherently requires the housing for the system to be wider and requires the operator to have to reach over at least one laser while performing set up or maintenance of the system. Therefore, it would be desirable to provide a light projection apparatus that eliminates the positioning of ion lasers at one or both sides of these components to provide convenient access for maintenance and/or operation of the system.